1. Field of the Invention
This invention relates to an infrared optical fiber used as an optical transmission line in a laser beam machine or a laser medical instrument, and to a method of manufacturing the infrared optical fiber. This invention also relates to an optical fiber cable using the infrared optical fiber.
2. Description of the Prior Art
In medical operations using laser cutting and laser beams, various kinds of laser beams are currently used according to the purpose of the operation. In the latter case, a carbon dioxide (CO.sub.2) laser beam is used because of its high absorptivity in the living organism and its superior incising and vaporizing capabilities.
As a means for transmitting the CO.sub.2 laser beam to the target, a mirror articulated optical waveguide using a combination of mirrors has previously been used, because quartz optical fibers as used for telecommunications cannot transmit the CO.sub.2 laser beam, which has a wavelength of 10.6 .mu.m and so falls within the category of medium infrared radiation.
However, since the mirror articulated optical waveguide lacks operatability for precise medical operations, many efforts have been made to replace it with an infrared optical fiber. As a result, some infrared optical fibers have been put into practical use for general surgery, but it still cannot be said that these infrared optical fibers have sufficient flexibility.
In particular, there has recently been a need for a so-called "CO.sub.2 laser endoscope" that enables the treatment of internal organs to be performed without an external medical operation for cutting open the body, by inserting the infrared optical fiber along with the endoscope into the interior of the body to direct the CO.sub.2 laser beam to the diseased part, but the current infrared optical fibers completely lack flexibility for this application. The infrared optical fiber used in this application is required to have sufficient flexibility so as to be easily bent within the interior of the body, as well as the power transmission capability for incising and vaporizing the diseased part. As to the conditions of use for the endoscopes currently used, it is considered that what is required in cases where an infrared optical fiber is repeatedly used many times is flexibility which can retain the power transmission capability against repeated flexing of approximately 10,000 times with a curvature radius of 20 mm. In cases where an infrared optical fiber is used only once or a few times at most, flexibility which confers the power transmission capability against repeated flexing of about 2000 times with a curvature radius of 20 mm on the infrared optical fiber is generally sufficient. It is also important for the infrared optical fibers to have excellent optical characteristics such as a transmittance and a radiation spread angle of the laser beam.
Metal halides such as thallium halide, silver halide and cesium halide have been previously used as infrared optical fiber materials which can transmit the CO.sub.2 laser beam with high efficiency, but these materials generally have inferior mechanical bending characteristics and are easily breakable. Since the infrared optical fiber formed from silver halide is relatively flexible and not easily breakable, various researches are currently underway for improvement. A report has been made of an example of infrared optical fibers formed from materials with improved elongation at rupture by adding 0.01 to 10 percent by weight of silver chloride to silver bromide or of silver bromide to silver chloride for improved mechanical bending characteristics. However, while the infrared optical fiber of this composition does not break with bending of a radius of as small as 20 mm because of its great elongation rate, there is the problem that it is susceptible to plastic deformation with bending because of its small yield strength, causing fusing at the deformed portion during transmission of the laser beam or resulting in degradation in the optical characteristics.
There has also been a report of an example of infrared optical fibers formed to have a diameter of 0.9 mm from materials with mechanical strength improved by using silver chloride and silver bromide in an equivalent mole ratio in its composition, but it has the problem that after a slight number of repetitive bending with a radius of 20 mm, it is fused during transmission of the laser beam; the optical characteristics deteriorate; and sometimes, the optical fiber ruptures.
Furthermore, with regard to the method of manufacturing infrared optical fibers, the conventional hot extrusion (c.f., FIG. 5) of metal halide materials has the problem that the infrared optical fibers are molded in a corrugated form and the extrusion speed is slow. This tendency is particularly noted in the extrusion of silver halide materials with improved mechanical strength, and therefore, the extrusion of optical fibers with excellent optical characteristics has not been possible.